1. Field of the Invention
The present invention relates to non-aqueous electrolyte secondary batteries having excellent high-temperature storage characteristics, high overcharge safety, and other advantageous characteristics.
2. Description of the Prior Art
Non-aqueous electrolyte secondary batteries have been widely used as the driving power sources of portable devices because of their high energy density and high capacity. In recent years, such batteries have also been used as the driving power sources of electric tools, electric vehicles, hybrid vehicles, and the like. In such uses, non-aqueous electrolyte secondary batteries are likely to be used and stored in a hot environment and to be charged at a high rate. Therefore, these batteries are required to have excellent high-temperature storage characteristics and to ensure safety against overcharge due to high-rate charging.
Conventional non-aqueous electrolyte secondary batteries use as a positive electrode active material a lithium-cobalt composite oxide (a lithium cobalt oxide). The use of the lithium-cobalt composite oxide, however, has a problem that a non-aqueous solvent reacts with the positive electrode and is decomposed to generate gas, thereby reducing the safety of the battery. To solve this problem, a different element (such as Zr or Mg) is added to the lithium-cobalt composite oxide so as to improve the structure stability of the positive electrode active material. This technique, however, is not enough to solve the problem.
Such conventional techniques related to non-aqueous electrolyte secondary batteries are disclosed in the following Patent Documents 1 to 5.
Patent Document 1: WO05/048391
Patent Document 2: Japanese Patent Unexamined Publication No. 2004-220952
Patent Document 3: Japanese Patent Unexamined Publication No. 2005-340080
Patent Document 4: Japanese Patent Unexamined Publication No. 2006-32301
Patent Document 5: Japanese Patent Unexamined Publication No. 2006-100262
The technique of Patent Document 1 uses a non-aqueous solvent containing a cyclic carbonate compound, a chain carbonate compound, or a cyclohexylbenzene compound in which one or two halogen atoms are bonded to a benzene ring. Patent Document 1 says this technique provides a battery having high overcharge safety and excellent cycle characteristics.
The technique of Patent Document 2 uses as a positive electrode active material a composite oxide expressed by LizCO1-x-yMgxMyO2 where M is at least one of Al, Ti, Sr, Mn, Ni, and Ca. Patent Document 2 says this technique provides a battery having excellent heat resistance during overcharge.
The technique of Patent Document 3 uses a non-aqueous electrolytic solution containing a compound expressed by Chemical Formula 1 below. Patent Document 3 says this technique provides a battery excellent in cycle characteristics, capacitance, and storage characteristics.

where R1 and R2 are independent of each other and each represents an alkyl group having 1 to 6 carbon atoms and being able to be branched; R3 and R4 each represents either a non-substituted methylene group or a methylene group containing an alkyl group having 1 to 4 carbon atoms; and X represents a vinylene group, a 2-butenylene group, or a 1,3-butadienylene group.
The technique of Patent Document 4 uses a non-aqueous solvent containing a carboxylic acid ester or a ketone in which a third alkyl group is directly bonded to a carbonyl group. Patent Document 4 says this technique provides a battery having excellent cycle characteristics.
The technique of Patent Document 5 uses a non-aqueous electrolytic solution containing a hydrogenated terphenyl. Patent Document 5 says this technique provides a battery having high safety against overcharge.
All these techniques, however, are still not sufficient in high-temperature storage characteristics or safety against overcharge due to high-rate charging.